Wide belt sanding machine

ABSTRACT

A wide belt sanding machine with a vertically oriented sanding head mounted for free up and down motion with any desired proportion of its weight borne by adjustable fluid pressure devices.

United States Patent 1191 Fair, Jr.

[ Jan. 14, 1975 WIDE BELT SANDING MACHINE [75] Inventor: Raymond G. Fair, Jr., Minneapolis,

Minn.

[73] Assignee: Timesavers, Inc., Minneapolis,

Minn.

[22] Filed: June 7, 1973 211 App]. No.: 367,976

[52] US. Cl. 51/147 [51] Int. Cl B24b 21/04 [58] Field of Search 51/135 R, 137, 138,139,

[5 6] References Cited UNITED STATES PATENTS 2,990,655 7/1961 Guinn 51/99 3,103,768 9/1963 Tsunoda et a1. 51/142 3,354,588 11/1967 Roehrig 51/141 3,498,004 3/1970 Hensley 51/99 FOREIGN PATENTS OR APPLICATIONS 692,909 8/1964 Canada 51/138 Primary ExaminerAl Lawrence Smith Assistant ExaminerNicholas P. Godici [57] ABSTRACT A wide belt sanding machine with a vertically oriented sanding head mounted for free up and down motion with any desired proportion of its weight borne by adjustable fluid pressure devices.

12 Claims, 5 Drawing Figures PATENTEB JAN 1 41975 SHEH 10E 3 PATENTED JAN 1 41975 SHEET 30F 3 WIDE BELT SANDING MACHINE This invention relates to sanding machines and, more particularly, to wide belt sanding machines designed to handle panels and other flat surfaced workpieces. The pending John M. Bernu application Ser. No. 240,683, filed Apr. 3, 1972 and assigned to the assignee of this invention, now Pat. No. 3,777,442, and the Roehrig U.S. Pat. No. 3,354,588 disclose sanding machines of the class to which this invention pertains.

Practically all wide belt sanding machines of that class have at least one sanding head consisting of a frame with a contact drum or roll and one or more idler rolls rotatably mounted therein, and an endless sanding belt trained about said rolls, one of the rolls (usually the contact drum) being power driven to impart high speed orbital motion to the belt.

The work to be sanded is carried through the machine by feed rolls or an endless conveyor belt, with its surface to be sanded in engagement with the contact drum-supported portion of the sanding belt.

Obviously, for the machine to perform its intended function, the sanding belt must engage the work under some degree of pressure. The magnitude of that pressure and the coarseness or grit size of the abrasive material that forms the active surface of the sanding belt are major factors governing the amount of material abraded from the work surface, or in other words, the depth of the cut taken during one pass of the work through the machine.

Where sizing or dimensioning of lumber is the objective, coarse grit size and engagement of the sanding belt with the workpiece under appreciable pressure is indicated; but, where a smooth finished surface is desired, a finer grit size and considerably less pressure in the engagement of the sanding belt with the work are essential. Moreover, since the ostensibly flat surface of a workpiece being sanded sometimes lacks that quality, the contact drum or its equivalent by which the sanding belt is held engaged with the workpiece surface must be capable of some degree of up and down motion or float if the surface of the workpiece throughout its entire area is to be uniformly finished, but since belt tension should remain constant, the entire sanding head must move up and down to gain that objective.

Attainment of that objective was the purpose of the invention disclosed and covered ing the Pendergast U.S. Pat. No. 2,876,600.

The machine of the Pendergast patent is known in the industry as a Floating Slant Head sander. It was so called because its sanding head instead of being vertically oriented, as in the more conventional sanding machines was tilted and disposed at an acute angle to the horizontal path of the work through the machine. The head was pivotally supported on the main frame of the machine to rock about a fulcrum so located with respect to the center of gravity of the sanding head that only a portion of its weight was borne by the workpiece to effect work performing engagement between the abrasive belt and the work surface.

To enable adjustment of the pressure with which that engagement was effected weights were slidably mounted on the sanding head for movement towards and away from the fulcrum about which it rocked.

With proper adjustment the machine of the Pendergast patent produced a very light sanding action, and by virtue of the teeter-totter mounting of its sanding head,

it allowed the contactdrum to rise and fall with the slightest variations in the elevation of the workpiece surface.

But the sanding machine of the Pendergast patent demanded considerable floor space because of the virtually horizontal disposition of its sanding head. it was also an expensive machine to build.

Although vertical disposition of the sanding head to reduce the floor space requirement was obvious enough, there were apparently insurmountable deterrents to taking that course. For several reasons it was believed that the results accomplished with the floating slant head sander could notbe duplicated with a vertically oriented sanding head.

First, the weight of the sanding head borne by the work could not be balanced as it was with the floating slant head design;

Second, the need to securely hold the sanding head against the thrust resulting from the advance of the work through the machine and the reaction between the fast-moving sanding belt and the work, and the inevitable cocking of the contact roll by variations in work performance along the length of the contact zone could be expected to cause binding in the guideways constraining the head to vertical motion, which in turn would interfere with the needed free up and down motion of the sanding head; and

Finally, there was the ingrained conviction in the industry that the hardening of the yieldable outer surface of the contact drum caused by belt tension would make it difficult if not impossible to do light finish sanding if that hardened portion of the drum held the sanding belt in engagement with the work as would be the case with the sanding head vertical, which of course was not the case in the floating slant head machine of the Pendergast patent, since in that machine the zone of contact between the belt and the work surface was not where the hardening of the contact drum surface occurred.

Notwithstanding these hurdles to the attainment of the attributes of the floating slant head design with a vertically oriented sanding head, the present invention has achieved that objective. lts success stems from mounting the sanding head in the main frame of the machine in a way which obviates the need for guide rails to constrain it to the desired up and down movement, and levitating or offsetting as much of the weight of the head as needed to reduce the pressure with which the sanding belt is engaged with the workpiece, to a magnitude suitable for light sanding.

These structural differences from the machine of the Pendergast patent, plus the discovery that belt tension need not be so great "as to significantly deform and harden the yieldable cylindrical contact drum surface, resulted in an important improvement in sanding ma chine, expecially in the area of cost reduction since, with this invention, the structure by which the floating mounting of the sanding head is obtained is simpler than in the floating slant head design and in addition it enables the attributes of the floating sanding head to be realized within the dimensions of the substantially less costly and more compact sander disclosed in the aforesaid Bernu application Ser. No. 240,683.

With these observations and objectives in mind, the manner in which the invention achieves its purpose will be appreciated from the following description and the accompanying drawings, which exemplify the invention, it being understood that changes may be made in the specific apparatus disclosed herein without departing from the essentials of the invention set forth in the appended claims.

The accompanying drawings illustrate one complete example of an embodiment of the invention constructed according to the best mode so far devised for the practical application of the principles thereof, and in which:

FIG. 1 is a view in side elevation of a sanding machine embodying this invention, parts thereof being broken away and in section;

FIG. 2 is a horizontal cross sectional view of the machine taken generally on the plane of the line 2-2 in FIG. 1;

FIG. 3 is a cross sectional view through the sanding head of the machine taken generally on the plane of the line 33 in FIG. 1;

FIG. 4 is a perspective view of those parts of the machine most directly affected by this invention; and

FIG. 5 is a diagrammatic side view of the sanding head showing the same equipped with a polishing platen in place of the contact drum to hold the sanding belt against the work.

Referring to the accompanying drawings the numeral 5 indicates generally the main frame of the sanding machine of this invention, and which includes spaced apart upright side walls 6 and 7. A conveyor bed 8 located between the side walls supports and carries the workpieces (WP) through the machine. This bed consists of a rigid frame 9 with a smooth surfaced platen 10 that forms its top and an endless conveyor belt 11 trained over rollers 12 and 13 journalled in the front and rear ends of the frame with its top stretch slidably supported by the platen. The rear roller 13 is driven by a motor 14 through av speed reducer to impart work feeding motion to the conveyor belt at a selected speed and from right to left in FIG. 1. Conventional pinch rolls 15 hold the work down on the conveyor belt.

Above the conveyor bed is the sanding head 16 of the machine. It is of generally conventional design and construction and has a rigid frame that includes a center bar 17 with side plates l818 welded to its ends, a contact drum l9 beneath the center bar journalled in bearings fixed with respect to the center bar, and an upper idler roll 20 above the center bar journalled in bearings that are mounted on a yoke 21 which is supported from the center bar by an air cylinder 22. The sanding belt 23 is of course trained about the idler roll and contact drum and is maintained under tension by pressure in the air cylinder 22.

As is customary, the cylindrical surface layer of the contact drum is yieldable being formed of rubber or its equivalent, so that an excessively tensioned sanding belt flattens and hardens that portion of the contact drum surface against which the tensioned belt is drawn. That portion of the contact drum surface is of course bisected by the median plane of the sanding head, and therefore is bottommost when the sanding head is vertically disposed. Since it has generally been deemed unwise to attempt to do light finish sanding without the cushion provided by the yieldability of the contact drum surface, vertical orientation of the sanding head was heretofore thought to be undesirable where light finish sanding was sought. However, it has been found that excessive belt tension is not needed in the machine of this invention. Accordingly the belt tensioning pressure in the air cylinder 22 is held to a value at which objectionable flattening and hardening of the contact drum surface does not occur, thereby making vertical disposition of the sanding head entirely feasible when it is mounted in the main frame of the machine in the way in which it is with this invention and which provides for free and entirely unhindered up and down motion or floating of the head with respect to the conveyor bed.

The desired work performing engagement between the sanding belt and the passing workpiece thus can be maintained despite variations in elevation of the top surface of the workpiece.

The structure by which the sanding head is floatingly mounted in the main frame of the machine includes a plate-like bracket 24 solidly bolted or otherwise secured to each of the side walls of the main frame. These brackets provide coaxial bearings for stub shafts 25 that project in opposite directions from a pair of arms 26-27. The arm 26 is solidly bolted or otherwise fixed to the center bar of the sanding head frame near the end thereof which is adjacent to the main frame side wall 7 and which, for convenience of description can be considered its inboard end. The other arm 27 is not directly connected to the sanding head frame. Instead it is detachably connected with the sanding head frame at the outboard end of its center bar, by means of a separable connector device indicated generally by the numeral 28 and which in function is like the outboard center bar support of the aforesaid Bernu application.

As best seen in FIG. 4, the arm 27 overlies the outer face of the main frame side wall 6 and at its free end portion, which is in line with the sanding head, there is a bracket 29. This bracket has an attaching flange 30 that is bolted to the outer face of the arm 27, and an upper flange 31, the top surface 32 of which is flat and horizontal. Confined between this top surface 32 and an opposing downwardly facing flat horizontal surface 33 formed by the bottom of a bracket 34 that is welded to the end plate 18 at the outboard end of the sanding head frame, is a removable spacer block 35.

A C-clamp 36 having upper and lower arms, has its upper arm pivotally connected as at 37 to a hand lever 38 which in turn is pivotally connected at 39 with the bracket 34. The lower arm of the clamp hooks under the upper flange 31 so that movement of the hand lever 38 to its erect position shown in FIG. 4 draws the bracket 29 upward. The dimensions of the C-clamp and the distance between the pivots 37 and 39 are such that an over-center toggle action takes place as the hand lever 38 is swung up to its erect position, to thereby tightly clamp the removable spacer block between the surfaces 32 and 33. With the spacer block thus gripped, the arm 27 is solidly connected with the sanding head.

A torque tube 40 with flanges 41 welded to its ends connects the arms 26 and 27 and constrains them to swing in unison about the fixed axis of the coaxial stub shafts 25, the torque tube being secured to the arms coaxially of said fixed axis, by having its flanges 41 bolted to the inner faces of the arms.

With the free ends of both of the arms 26-27 solidly connected to the sanding head frame, the frame and the arms form a rigid unitary structure that can rock freely about the fixed axis defined by the coaxial bearings in which the stub shafts 25 are journalled. Since the fixed pivot axis is at the same or nearly the same level as the axis of the contact drum, rocking of this rigid unitary structure about its fixed pivot or hinge axis results in very nearly straight line up and down motion of the sanding head and of course the bottommost part of the sanding belt orbit. Therefore, in the absence of any restraint the full weight of the sanding head is available to press the sanding belt against the workpiece on the conveyor bed.

However, since only a relatively light a sanding pressure is desired, the weight of the sanding head is largely borne by a pair of air cylinders 42-42 one at each side of the machine. By controlling the pressure in these cylinders any desired contact pressure can be established and maintained between the sanding belt and the work surface, even though the elevation of the work surface may vary as the workpiece moves through the machine since the manner in which the sanding head is mounted allows it to float freely with variations in the work surface elevation.

The cylinders 42-42 have their closed ends pivotally connected to the side walls of the main frame by mounting brackets 43 that are solidly fixed to these walls, and the pistons in the cylinders have their rods 44-44 connected with the sanding head. At the side of the machine at which the inboard end of the sanding head is located the piston rod identified by the numeral 44 is pivotally connected directly to the sanding head through a bracket 45 that is fixed to and extends down from the adjacent inboard end of the center bar; but at the other side of the machine the piston rod 44' is not directly connected to the sanding head. Instead it is pivotally connected as at 46 to a lug 47 that forms part of the bracket 29. As a result the connection between the piston rod 44' and the sanding head is effected through the connector device 28 and hence is detachable or separable by the simple expedient of swinging the hand lever 38 from its erect position whereupon the C-clamp can be unhooked from the flange 31 and the space block 35 removed.

As will no doubt be obvious, the need for having the sanding belt removable to permit a worn belt to be re placed with a new one, necessitates having the connection between the arm 27 and the outboard end of the sanding head separable, and for the same reason the free end portion of the arm 27 which extends beyond the median plane of the sanding head, must be below the level of the bottommost part of the sanding belt orhit. And of course there must be nothing at the outboard end of the sanding belt to obstruct the space that results from removal of the spacer block 35, it being understood that the C-clamp38 is swung out of the way when a sanding belt is removed or placed in position, which of course requires that the belt tensioning pressure in the cylinder 22 be relaxed.

Inasmuch as the main frame and the sanding head frame are weldm ents so that close tolerances cannot very well be achieved in the positional realtionship of the various machine parts, and since it is desired to use a spacer block of a given thickness, some adjustability of the spacing between the surfaces 32 and 33 between which the block is clamped is needed. That adjustability is obtained by providing the flange 30 of the bracket 29 with vertically elongated slots through which its attaching bolts pass. By that simple expedient, the elevation of the upwardly facing surface 32 can be accurately related to the downwardly facing surface 33 to assure against imposing any twisting stresses in the unitary structure consisting of the sanding head and the arms 2627 as the clamping device is actuated to connect the outboard end of the sanding head to the arm 27.

Inasmuch as some contact pressure must always be available, the pressure in the cylinders 4242 is never so great that the entire weight of the sanding head is borne thereby. Accordingly, in the absence of some limitation to the descent of the sanding head, the

bottommost portion of its sanding belt would contact the conveyor belt when no workpiece was present. To avoid that obviously objectionable consequence the dc scent of the sanding head is limited by a stop screw 50 adjustably fixed with respect to the main frame by being threaded in a lug 51 that projects from the bracket 24 at the inboard side of the machine, in position to have the adjacent arm 26 seat thereon.

The sanding belt is driven by a motor 52 that is driv ingly connected with the contact drum through a belt 53, and since the motor is mounted coaxially with the fixedaxis about which the sanding head rocks as it rises and falls with variations in the elevation of the top surface of the workpiece, the drive to the contact drum is in nowise affected by such up and down floating of the head.

To accommodate different workpiece thicknesses, the conveyor bed is vertically adjustably mounted through the use of jack screws 54 in the conventional way, the adjustment being effected by means of a hand wheel 55.

Although the invention has been described as applied to a sanding machine in which a rotating contact drum holds the sanding belt against the work, it is equally applicable to machines in which a stationary polishing platen or shoe performs this function. Thus as diagrammatically shown in FIG. 5, the sanding head may have a platen 56 mounted at theunderside of its center bar and flankedby rolls 57 and 58, the former being power driven to impart orbital motion to the sanding belt. In other respects the machine equipped with a sanding head as depicted in FIG. 5 would be the same that of FIGS. 1-4.

As will no doubt be readily apparent to those skilled in the art to which this invention pertains, a significantly improved sanding machine has been achieved thereby. Not only have the advantages of a free floating sanding head coupled with a wide range of easily adjusted contact pressure been accomplished with a simpler less costly machine, but in addition, as compared to the slant head design of the prior art, with the machine of this invention, the mouth or inlet of the dust collector with which the machine is equipped (though not illustrated in the drawings) can be more strategically located, that is, closer to the zone of action.

Also, those skilled in the art will appreciate that the invention can be embodied in forms other than as herein disclosed for purposes of illustration.

I claim:

1. A sanding machine comprising:

A. a main frame;

B. work conveying means in the main frame to carry work through the machine along a defined horizon- 2. a frame including a horizontally oriented center bar having an inboard end and an outboard end, and

3. belt guiding and carrying means mounted above and below the center bar on which the endless sanding belt is removably mounted and by which the belt is constrained'to travel in a vertically oriented generally oval shaped orbit, said belt guiding and carrying means including a backup member by which the sanding belt as it traverses the bottommost portion of said generally oval shaped orbit is pressed against a workpiece being carried through the machine by said work conveying means;

D. mounting means by which the sanding head is connected with the main frame of the machine for bodily up and down motion so that the weight of the sanding head applied through said backup member presses the sanding belt against the workpiece and the distance between the bottom of the orbit of the belt and the work conveying means can vary to accommodate variations in the level of the workpiece surface being sanded, said mounting means comprising:

l. pivot means on the main frame of the machine providing a pair of coaxial spaced apart fixed pivots, the common axis of which is parallel with the center bar of the sanding head, and lies outside the orbit of the sanding belt at an elevation between the top and bottom of the sanding head,

2. a pair of arms, each pivotally connected near one end thereof with one of said pivot means so that the free ends of said arms swing about said fixed axis,

3. means rigidly connecting one of said arms with the frame of the sanding head at a location near the inboard end of its center bar and laterally outward of the adjacent edge of the endless sanding belt, the other of said arms having its free end located below the level of the bottommost portion of said generally oval shaped orbit of the sanding belt so as not to interfere with removal and replacement of the belt, and

4. means forming a readily separable rigid connection between the free end of said other arm and the outboard end of the center bar; and

E. adjustable load supporting means reacting between the sanding head and the main frame of the machine to support part of the weight of the sanding head and thereby govern the pressure with which the sanding belt engages the work.

2. The sanding machine of claim 1, wherein said readily separable connection includes a removable spacer in line with the bottommost portion of the orbit of the sanding belt, removal of which provides space through which the belt can be passed during removal and replacement thereof.

3. The sanding machine of claim 2, wherein said readily separable connection further comprises:

A. a member rigid with and projecting from the center bar of the sanding head, said member having a downwardly facing flat horizontal surface;

B. a member rigidly fixed to the adjacent arm and having an upwardly facing flat horizontal surface beneath said downwardly facing flat horizontal surface and spaced therefrom a distance substantially equal to the thickness of said removable spacer,

the spacer being removably positioned between said surfaces; and

C. clamping means for drawing said members towards one another to thereby clamp the spacer be tween said surfaces,

said last named means being separable from one of said members and when separated therefrom being movable to a position in which it does not obstruct passage of the sanding belt through the space which results from removal of the spacer.

4. The sanding machine of claim 3, wherein said clamping means comprises:

A. a lever pivoted to one of said members, and

B. a hook pivoted to said lever and adapted to be engaged with the other member and to draw said members towards one another as the lever is swung in one direction about its pivot.

5. The sanding machine of claim 1, further characterized by connecting means rigidly connecting said pair of arms and constraining them to swing in unison about said fixed axis.

6. The sanding machine of claim 5, wherein said connecting means comprises a torque tube coaxial with said fixed axis.

7. The sanding machine of claim 1, wherein said adjustable load supporting means comprises a pair of fluid pressure devices, one at each side of the sanding head.

8. The sanding machine of claim 7, wherein said fluid pressure devices are of the cylinder and piston type and further characterized by:

means pivotally connecting each cylinder to the main frame;

means non-detachably connecting the piston of one of the cylinders with the inboard end of the center bar of the sanding head; and

means detachably connecting the piston of the other cylinder with the outboard end of the center bar of the ssanding head, detachment of said last mentioned connection providing a space in line with the adjacent portion of the sanding belt orbit through which the belt may be passed.

9. The sanding machine of claim 1, wherein said backup member is a rotatable contact drum.

10. The sanding machine of claim 1, wherein said backup member is a polishing platen extending along the bottom of the sanding head and flanked by a pair of rolls, one of which is power driven and drives the sanding belt.

11. The sanding machine of claim 9, further characterized by drive means for the sanding belt comprising a motor mounted on the main frame of the machine with its axial coaxial with said fixed axis about which the sanding head is swingable; and drive transmission means connecting the motor with the contact drum.

12. The sanding machine of claim 10, further characterized by drive means for the sanding belt comprising a motor mounted on the main frame of' the machine with its axis coaxial with said fixed axis about which the sanding head is swingable; and drive transmission means connecting the motor with said driven pair of rolls.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,859 ,758 Dated January 1'4 1 1975 Inventor(s) Raymond G. vFail", Jr.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line H7 "ing" should read --in-. Column 5 line 9: The word "a" should be omitted I after "light" line #9: "belt" should read --head-- Column 6, line 6: "42'-'+2"' should read }2- 2'- Column 8, line +1: "ssanding" should read .'-sandi'ngline 55: "axial" should read --axis-- Signed and sealed this" day of April 1.975.

(SEAL Attest:

' a 4 C. E-IARSHALL DANN- RU'I'Z-i C. ZIASOI! v Commissioner of Patents attesting Officer and Trademarks -'ORM PO-IOSO (1M9) 

1. A sanding machine comprising: A. a main frame; B. work conveying means in the main frame to carry work through the machine along a defined horizontal path; C. a vertically oriented sanding head above said defined path having
 1. an endless sanding belt,
 2. a frame including a horizontally oriented center bar having an inboard end and an outboard end, and
 3. belt guiding and carrying means mounted above and below the center bar on which the endless sanding belt is removably mounted and by which the belt is constrained to travel in a vertically oriented generally oval shaped orbit, said belt guiding and carrying means including a backup member by which the sanding belt as it traverses the bottommost portion of said generally oval shaped orbit is pressed against a workpiece being carried through the machine by said work conveying means; D. mounting means by which the sanding head is connected with the main frame of the machine for bodily up and down motion so that the weight of the sanding head applied through said backup member presses the sanding belt against the workpiece and the distance between the bottom of the orbit of the belt and the work conveying means can vary to accommodate variations in the level of the workpiece surface being sanded, said mounting means comprising:
 1. pivot means on the main frame of the machine providing a pair of coaxial spaced apart fixed pivots, the common axis of which is parallel with the center bar of the sanding head, and lies outside the orbit of the sanding belt at an elevation between the top and bottom of the sanding head,
 2. a pair of arms, each pivotally connected near one end thereof with one of said pivot means so that the free ends of said arms swing about said fixed axis,
 3. means rigidly connecting one of said arms with the frame of the sanding head at a location near the inboard end of its center bar and laterally outward of the adjacent edge of the endless sanding belt, the other of said arms having its free end located below the level of the bottommost portion of said generally oval shaped orbit of the sanding belt so as not to interfere with removal and replacement of the belt, and
 4. means forming a readily separable rigid connection between the free end of said other arm and the outboard end of the center bar; and E. adjustable load supporting means reacting between the sanding head and the main frame of the machine to support part of the weight of the sanding head and thereby govern the pressure with which the sanding belt engages the work.
 2. The sanding machine of claim 1, wherein said readily separable connection includes a removable spacer in line with the bottommost portion of the orbit of the sanding belt, removal of which provides space through which the belt can be passed during removal and replacement thereof.
 2. a frame including a horizontally oriented center bar having an inboard end and an outboard end, and
 2. a pair of arms, each pivotally connected near one end thereof with one of said pivot means so that the free ends of said arms swing about said fixed axis,
 3. means rigidly connecting one of said arms with the frame of the sanding head at a location near the inboard end of its center bar and laterally outward of the adjacent edge of the endless sanding belt, the other of said arms having its free end located below the level of the bottommost portion of said generally oval shaped orbit of the sanding belt so as not to interfere with removal and replacement of the belt, and
 3. belt guiding and carrying means mounted above and below the center bar on which the endless sanding belt is removably mounted and by which the belt is constrained to travel in a vertically oriented generally oval shaped orbit, said belt guiding and carrying means including a backup member by which the sanding belt as it traverses the bottommost portion of said generally oval shaped orbit is pressed against a workpiece being carried through the machine by said work conveying means; D. mounting means by which the sanding head is connected with the main frame of the machine for bodily up and down motion so that the weight of the sanding head applied through said backup member presses the sanding belt against the workpiece and the distance between the bottom of the orbit of the belt and the work conveying means can vary to accommodate variations in the level of the workpiece surface being sanded, said mounting means comprising:
 3. The sanding machine of claim 2, wherein said readily separable connection further comprisEs: A. a member rigid with and projecting from the center bar of the sanding head, said member having a downwardly facing flat horizontal surface; B. a member rigidly fixed to the adjacent arm and having an upwardly facing flat horizontal surface beneath said downwardly facing flat horizontal surface and spaced therefrom a distance substantially equal to the thickness of said removable spacer, the spacer being removably positioned between said surfaces; and C. clamping means for drawing said members towards one another to thereby clamp the spacer between said surfaces, said last named means being separable from one of said members and when separated therefrom being movable to a position in which it does not obstruct passage of the sanding belt through the space which results from removal of the spacer.
 4. means forming a readily separable rigid connection between the free end of said other arm and the outboard end of the center bar; and E. adjustable load supporting means reacting between the sanding head and the main frame of the machine to support part of the weight of the sanding head and thereby govern the pressure with which the sanding belt engages the work.
 4. The sanding machine of claim 3, wherein said clamping means comprises: A. a lever pivoted to one of said members, and B. a hook pivoted to said lever and adapted to be engaged with the other member and to draw said members towards one another as the lever is swung in one direction about its pivot.
 5. The sanding machine of claim 1, further characterized by connecting means rigidly connecting said pair of arms and constraining them to swing in unison about said fixed axis.
 6. The sanding machine of claim 5, wherein said connecting means comprises a torque tube coaxial with said fixed axis.
 7. The sanding machine of claim 1, wherein said adjustable load supporting means comprises a pair of fluid pressure devices, one at each side of the sanding head.
 8. The sanding machine of claim 7, wherein said fluid pressure devices are of the cylinder and piston type and further characterized by: means pivotally connecting each cylinder to the main frame; means non-detachably connecting the piston of one of the cylinders with the inboard end of the center bar of the sanding head; and means detachably connecting the piston of the other cylinder with the outboard end of the center bar of the ssanding head, detachment of said last mentioned connection providing a space in line with the adjacent portion of the sanding belt orbit through which the belt may be passed.
 9. The sanding machine of claim 1, wherein said backup member is a rotatable contact drum.
 10. The sanding machine of claim 1, wherein said backup member is a polishing platen extending along the bottom of the sanding head and flanked by a pair of rolls, one of which is power driven and drives the sanding belt.
 11. The sanding machine of claim 9, further characterized by drive means for the sanding belt comprising a motor mounted on the main frame of the machine with its axial coaxial with said fixed axis about which the sanding head is swingable; and drive transmission means connecting the motor with the contact drum.
 12. The sanding machine of claim 10, further characterized by drive means for the sanding belt comprising a motor mounted on the main frame of the machine with its axis coaxial with said fixed axis about which the sanding head is swingable; and drive transmission means connecting the motor with said driven pair of rolls. 